One of the most well-known ideas introduced by Plato is "truth is justified true belief". This appears in Theaetetus as the third theory of what knowledge is which is associated with belonging to Plato.
What is true must be known by the subject, they must believe it. The Sophists appeared to agree with that, and so would relativists today. "Man is the measure of all things" Protagoras is supposed to have said. The Sophists instead of dialectic taught rhetoric, to win the argument through appearances to change the mind of opponent, not to discover the truth or continue the discussion. They did this often for payment, seeing knowledge's value as instrumental.
But belief isn't enough says Socrates. Belief must have external support. The English word truth derives from treowth which means something like good faith. What does good mean, if it isn't just having faith? It is external. But more than that; what is external must somehow also be linked to what is good.
In The Republic book I, Socrates gets Thrasymachus the critic of the utility of justice to accept that the good for any person requires knowledge. A good doctor must have knowledge of medicine and of the human body to be a good doctor. Anything that exists has a nature to it. Living to its full potential is what defines its own good. If one hand does not move, it is not a good watch as it cannot tell accurate time which is what a watch is supposed to do. The good of something entails knowledge of what that thing is.
For something to be good it must serve some end. The good of a watch for me is that it can prevent me from being late to work among other things. In general the end of anything, the highest good, is happiness. It must be that for the "soul" or mind that it has its own nature that defines its own good which is our highest good. The soul or mind's nature is that which superintends our own actions. All of our actions have the end of our happiness. The good of the soul is to command our actions to the fulfillment of our happiness according to the nature of what we happen to be commanding for that purpose. Thus it is that knowledge requires belief of what is true, of the nature of what we hold beliefs of. (This is essentialism, that the essence of something is the criterion of true statements). This is Plato's specific view of what makes something true. Even if one does not accept essentialism, knowledge still can scrutinize the patterns of our own thinking and the biases within.
To have knowledge is to have belief which is also justified by what is true. What is justification? It is to have a reason for a belief.
The Apology lays ground for why knowledge must be justified. Socrates tells the story of a friend who asked the Oracle of Delphi who is the wisest person. The Oracle answered that it was Socrates. Socrates was puzzled by this as surely other people know more than he does on plenty of things. But there is something that he knows that most others do not. That is he knows he knows nothing. Most people think they know something, where they really just have an opinion or prejudice. Socrates is the wisest because he admits he does not think he knows anything, which is the source of hubris.
Elsewhere in Plato's dialgogues such as Meno is the paradoxical idea that one already has to know something in order to know it. That is have the potential or capacity for knowledge. The task of philosophy is to discern whether one really does in fact already know what they assume to know.
This need for justification is where the dialectic comes from. A belief is presented, assumed to be true. Then another proposed truth which is acknowledged to be believed by both sides. Then the principle of non-contradiction, over whether it is possible to believe these different things to be true according some common non-conflicting notion of truth. This cross examination begins with the belief and does not end until the belief stands up to asserted criticisms or is denied or changed.
Just as with Plato's tripartite definition of knowledge as true justified belief, the dialectic asserts belief, demands justification, and ends with the truth contained within this belief which may be naught.
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